Many barriers exist to minimizing the form factor of optical disk drives. For example, conventional optical disk drives such as a CD-ROM drive are configured for use with “second surface” optical disks. In a second surface optical disk, the information layer is covered by a relatively thick protective layer or substrate that is hundreds of microns in thickness. Considering that conventional laser light used to read and write in optical drives has a wavelength in the range of from around 400 to 800 nanometers, the relatively thick protective layer is thus many wavelengths in thickness. As such, imperfections such as scratches, dust, and fingerprints on the surface of the protective layer are defocused with respect to the underlying information layer. In this fashion, CD-ROMs and other second surface disks may be handled by users and exposed to dusty environments without needing a protective cartridge.
Although the use of second surface disks provides this advantageous defocusing property, it is also accompanied by certain drawbacks. For example, the relatively thick protective layer covering the information layer introduces significant optical aberrations and wave front distortions. In turn, these optical problems place a floor on the achievable feature size in the information layer, thereby limiting data capacity. However, as the optical disk size is reduced, it is important to minimize feature size in the information layer to provide significant data storage capability despite the presence of a relatively small information layer area. To achieve a significant data capacity within a small form factor optical disk drive, the present assignee has developed first surface optical disks such as disclosed in U.S. Ser. No. 10/891,173, filed Jul. 13, 2004, which is a divisional application of U.S. Ser. No. 09/315,398, filed May 20, 1999, now abandoned, the contents of both applications being incorporated by reference herein in their entirety. In these first surface disks, an information layer covers a substrate, which may be formed to define one or both of a read-only and a writeable area. Advantageously, the information layer may be formed from a continuous phase-change material such as, for example, an SbInS or GeTe—Sb2Te3-Sb so that the formation of the read-only and writeable areas (if both exist) requires no masking or other complicated manufacturing processes. The surface of the information layer may be covered with an optical coupling layer formed from a sputtered dielectric such as silicon oxynitride or a spin-coated-high-refractive-index nano-particle dispersed material for instance. The optical coupling layer does not introduce the aberrations and wave front distortions that the protective layer in second surface optical disks does such that the feature size may be substantially reduced. In this fashion, a significant data capacity is achieved despite the presence of a small form factor.
The present assignee also developed a small form factor optical disk drive for use with the inventive first surface optical disks. For example, U.S. Ser. No. 09/950,378, filed Sep. 10, 2001, discloses an optical disk drive having an actuator arm with an optical pick-up unit (OPU) mounted on one end. Turning now to FIG. 1, an exemplary actuator arm 104 is illustrated. The actuator arm includes an optical pickup unit (OPU) 103 at one end. By rotating about an axis B through a spindle 105, the actuator arm may move the OPU radially with respect to an optical disk (not illustrated). As used herein, radial movement is defined as movement parallel to an optical disk surface. Thus, to maintain tracking of an optical disk by the OPU, a tracking servo will command a desired radial displacement of the actuator arm. By flexing the actuator arm about an axis A, the OPU may move axially with respect to an optical disk to achieve a desired focus with regard to a projected laser beam from a lens 90. As used herein, axial movement is defined as movement transverse to an optical disk surface. Thus, to maintain focus, a focus servo will command a desired axial displacement of the actuator arm. By providing an actuator arm having these properties, a small form factor optical disk drive may be implemented. For example, the height of a disk drive incorporating this actuator arm may be as little as 10.5 mm. However, note that the OPU is aligned such that its height dimension is normal to or in the axial direction with respect to an optical disk surface. Thus, the overall achievable height reduction of such a drive architecture is limited by the thickness of the optical disk and its cartridge as well as height of the OPU (as measured from the bottom of OPU to the focused laser spot at the disk surface).
Additional height reduction may be achieved using the split-optics (which may also be denoted as a “sled-based”) architecture disclosed in U.S. application Ser. No. 11/052,367, filed Feb. 7, 2005,the contents of which are hereby incorporated by reference in their entirety. As seen in FIG. 2 and in the exploded view of FIG. 3, an optical pick-up unit (OPU) 200 is attached within a sled 205. Any suitable OPU design may be used, such as that discussed in U.S. application Ser. No. 09/950,378. However, note that a height dimension H for OPU 200 now lies in the radial plane with respect to a corresponding optical disk (not illustrated). In contrast, dimension H for OPU 103 was in the axial plane, or normal to the optical disk surface. Thus, the overall height of an optical disk drive using the sled-based architecture of FIG. 2 may be substantially reduced with respect to that provided by a system incorporating the actuator arm of FIG. 1.
As is conventional in a split-optics-based architecture, coarse tracking may be achieved by movement of the sled on rails. For example, the sled may be mounted on rails (not illustrated) through apertures 210 and bearing 220. As the sled is displaced on these rails, a beam projected by a lens 340 will move radially across the optical disk (not illustrated), thereby changing track locations. In addition, the lens may be displaced by a two-dimensional actuator (not illustrated) that may either radially or axially displace the lens with respect to the optical disk as necessary for fine tracking and focusing purposes. With respect to focusing, the lens acts in the far field in that it is many wavelengths removed from the corresponding optical disk. There is a limit to the effective numerical aperture that can be obtained in such a system. In turn, this limit places a limit on the achievable data density on the corresponding optical disk, a limit that is exacerbated in a small form factor system.
To address the need in the art for improved small form factor optical disk drives, a SIL-based drive is disclosed in U.S. application Ser. No. 11/148,140. The focused spot size is reduced because the focused spot is formed inside a SIL having an index of refraction n. For example, the SIL may be implemented using a high index of refraction material such as GaP, which has an index of refraction (n) of 3.3 at the red light wavelength used in conventional DVD players. Note that the areal data density in inversely proportional to the squared value of the spot size, which is proportional to n2 for a type of SIL that may be denoted as a “simple” SIL. A simple SIL comprises a sphere sliced at it's midpoint to form a hemisphere. A simple SIL formed from GaP having an index of refraction of 3.3 provides over an order of magnitude data capacity gain with respect to a conventional far field optical disk drive operating at the same wavelength. The data capacity gain may be further increased using a type of SIL lens that may be denoted as a “super” SIL. Whereas a simple SIL comprise a hemisphere, a “super” SIL may be formed by slicing a sphere of radius r and index n at a distance (r/n) below it's mid point diameter such that a beam converging toward a point at a distance (r/n) below the mid point is focused to the (r/n) point without introducing aberrations. The spot size is proportional to n4 in a super SIL. Thus, a super SIL formed from GaP having an index of refraction of 3.3 provides over two orders of magnitude data capacity gain with respect to a conventional far field optical disk drive operating at the same wavelength.
Regardless of whether a SIL or super SIL is used to reduce the focused spot size, the resulting solid immersion lens must be carefully aligned with an objective lens. For example, a spacer-based approach to mount an objective lens 410 to a SIL 400 etched from a wafer 405 combination is illustrated in FIG. 4. The objective lens is mounted within flanges 411 to a spacer 415 having a conical aperture. The wafer is aligned with the spacer such that the SIL is centered in the conical aperture. The objective lens refracts light such that each light ray 420 arrives substantially normally to the hemispherical surface of the SIL. In this fashion, the SIL simply functions not to refract the light rays but to provide a higher index of refraction. It will be appreciated, however, that an aspheric SIL may also be implemented. In general, rather expensive positioning equipment is necessary to ensure the proper alignment of the SIL/spacer/objective lens combination. Once the desired optical alignment is achieved the spacer/flange and wafer/spacer interfaces may be secured using optical adhesive. The resulting SIL/objective lens assembly may then be mounted for example, as the lens 340 in the sled of FIG. 2. Although an advantageously compact and high-density storage engine may then be achieved, the alignment of the SIL and objective lens introduces manufacturing cost. As an alternative to a separate objective lens/SIL combination, the wafer may be etched such that a hybrid solid immersion/objective lens (SIOL) is achieved. However, the amount of sag (depth of etching required) is quite difficult to achieve for such a combination lens.
Accordingly, there is a need in the art for improved SIOL designs as well as optical storage engines that incorporate such improved lenses.